1. Field of the Invention
The present invention concerns a method for phase-sensitive magnetic resonance imaging in which magnetic resonance data are acquired using an echo imaging sequence, as well as a magnetic resonance system for implementing such a method.
2. Description of the Prior Art
Magnetic resonance tomography (MRT) is an imaging modality that enables the acquisition of two-dimensional or three-dimensional image data sets that can depict structures inside an examination subject (even soft tissue) with high resolution. In MRT, protons in the examination subject are aligned in a basic magnetic field (B0) so that a macroscopic magnetization (alignment) of the protons exists, and the protons are subsequently excited by the radiation of RF (radio-frequency) pulses. The decay of the excited magnetization is subsequently detected by means of one or more induction coils, and a spatial coding of the acquired signal is achieved by the switching (activation) of slice selection, phase coding and frequency coding gradients before or during the acquisition. The acquisition of the decay signals thereby regularly takes place with a quadrature detection so that both the phase and the amplitude of the signal are detected. The signals detected in positional frequency space (domain) (k-space) can accordingly be represented as complex numbers and can be transformed by means of a Fourier transformation into image space (domain), in which phases and magnitudes can now be determined with spatial resolution.
In many imaging methods, only the magnitude of the complex image data is used to create an intensity image. The phase information is discarded. Furthermore, a combination of the magnitude data that were acquired with different coils is not optimal with regard to the signal-to-noise ratio (SNR).
For example, in conventional T2* (observed spin-spin relaxation time) or R2* (01/T2*) imaging, the magnitudes of three or more images are considered that were acquired at different echo times (TEs) from an individual proton species (type) (for example by the use of fat suppression). The T2* time can subsequently be determined with spatial resolution by the adaptation of a decay function to the magnitudes in the image data. However, this approach is very time-consuming and correspondingly prone to movement artifacts.
Other imaging methods use the acquired phase information. For example, the differences of the magnetic susceptibility of different tissue lead to phase differences. In susceptibility-weighted imaging (SWI), an expanded contrast signal image is thereby generated from the acquired magnitude and phase information, which expanded contrast signal image in particular has a contrast dependent on the oxygen content of the blood. Additional examples generally include phase contrast imaging as well as proton resonance frequency (PRF) shift thermometry. In the latter, a shift of the phase in acquired phase images is detected that is caused by a shift of the proton resonance frequency due to a temperature change.
In addition to these phase shifts with information content, there is a series of effects that cause unwanted phase shifts that can conceal usable information. These effects include, among other things, an inhomogeneity of the static B0 field, the susceptibility of articles and materials in proximity to the patient, phase shifts of the radiated RF pulses, and errors in the chronology of the acquisition sequence. Phase shifts that can be different for different reception coils can also occur in the acquisition chain.
These phase shifts make it difficult to compare and combine image data acquired at different echo times with one another. In particular, the combination of MR data acquired with different reception coils while acquiring phase information turns out to be difficult since each acquisition channel has a different phase shift. Objects within the examination subject—for example air bubbles, implants, needles or the like—can also lead to susceptibility artifacts, and thus also to phase shifts.
It is accordingly desirable to combine magnetic resonance (MR) data acquired for different echo times or with different coils so that the signal-to-noise ratio is improved and usable phase information is retained. In order to enable shorter scan durations, the acquisition method should also be capable of enabling such a combination for accelerated acquisition techniques and multi-echo imaging sequences. Moreover, the data should be able to be combined in a well-defined manner in order to be able to make reasonable conclusions about the images represented by the data acquired in such a manner.
A method known for R2* imaging is k-TE GRAPPA, which is described in detail in “k-TE Generalized Autocalibrating Partially Parallel Acquisition (GRAPPA) fur Accelerated Multiple Gradient-Recalled Echo (MGRE)R2* Mapping in the Abdomen”, by Xiaoming Yin et al., Magnetic Resonance in Medicine 61:507-516 (2009) P. 507-516. The method uses a partially parallel imaging method (GRAPPA) in combination with a view-sharing method in which missing k-space lines in incompletely scanned k-space are reconstructed on the basis of k-space lines acquired with adjacent coils and temporally adjacent sequences. The result of the method is a series of images of different echo times (TE), wherein the image noise in the image data varies spatially due to the reconstruction process. How the acquired image data can be combined optimally with regard to SNR is not disclosed in this publication.
Given a combination of image data under consideration of the phase, in conventional methods the phases of adjacent image points are compared in order to produce a total phase estimation. Image points with large phase variation—for example in regions with low SNR or along tissue boundaries—can interfere with the phase correction method. Furthermore, methods are known from U.S. Pat. No. 7,227,359 (for example) that are based on phase gradients in the image data and that implement a region expansion (region growing) to determine the phases using a seed image point.
Given image data that were acquired with a multi-echo imaging sequence—for example with a single-shot or EPI (echoplanar imaging) sequence—a “characteristic” echo time is normally associated with the data. This characteristic TE is typically the TE with which central k-space lines were scanned. However, in such sequences, different spatial frequencies (k-space lines) are scanned using different TEs, such that these respectively contribute to an error in the reconstructed image data depending on the respective TE. The errors in the phase accordingly depend on the acquisition sequence and moreover have a spatial dependency, such that they can only be predicted with difficulty.